This research project will investigate risk factors associated with the occurrence of Kaposi's Sarcoma (KS) in men aged 20-59 years. An epidemic of KS among relatively young homesexual men in New York City and California was reported recently by the present investigators and other (MMWR 30:305-308, 1981). Patient histories and clinical tests suggest that the number of etiologic hypotheses should be pursued epidemiologically, including drug and sex practices, disease histories, and exposure to chemicals, viruses, or other KS cases. We will administer a questionnaire to explore each of these issues among KS cases and age- and race- matched homosexual controls. Cases will be KS patients diagnosed or treated at New York University Medical Center. Controls will be drawn from gay practices of private physicians. Two controls per case will be interviewd and blood samples drawn from each control for B- and T- cell enumerations, T-cell subset typing, lymphocyte function studies, measurement of serum immunoglobulin levels, and measurement of antibody titers to cytomegalovirus, Epstein-Barr and other viruses. Univariate and multivariate logistic analysis will be used to statistically analyse the data with the goal of determining the etiology of the KS epidemic in homosexuals. Concurrently we will conduct a case-control study of KS in males and females aged 18-50 years diagnosed in New York area hospitals in the period 1965-1979, prior to the present epidemic. The goal of this study will be to better determine the onset of the present epidemic and to search for common risk factors between epidemic and endemic KS.